The invention relates to a method, a hydraulic system control unit, a hydraulic system and a working machine for controlling a hydraulic system adapted to perform at least one hydraulic work function in a working machine.
In several types of working machines with hydraulic systems, load sensing hydraulic systems have been introduced. In so-called open-center systems, i.e. hydraulic systems without load sensing, the hydraulic flow is load dependent and thus depends on system pressure or the load acting on the cylinder. FIG. 3 illustrates how a certain valve opening leads to hydraulic flow for a given load. As shown in FIG. 3, for a given constant valve opening X, increased load results in a reduced hydraulic flow. In order to achieve the same flow at increased load, the operator had to increase the angle of the hydraulic lever.
In load sensing systems the relationship between valve opening and hydraulic flow is independent of the load, as shown by line X in FIG. 4. This results in that the operator no longer has to compensate for the increased load by increasing the lever angle. The operator can keep the lever steady at a certain angle and the system will make sure to keep the flow steady.
A disadvantage with load sensing systems is that the operator no longer receives the feedback by having to compensate the increased load by increasing the lever angle. In certain situations, like e.g. handling of large rocks, such a feedback is actually wanted. With a load sensing system keeping the flow constant, the operator (being used to flow reduction) will not feel the weight of the load and thus might handle the machine less intuitively.
It is desirable to provide a method for controlling the hydraulic system that reduces the operators' negative experiences of load sensing systems, while at the same time retaining its advantages.
According to an aspect of the present invention, a method is provided for controlling a hydraulic system adapted to perform at least one hydraulic work function in a working machine. The hydraulic system performs the hydraulic control function in accordance with an operation signal determined by a hydraulic system control unit. In a first step the control unit receives an operator control input associated to said work function. The method is particularly characterized in a second step where the control unit receives a load input indicative of a load associated to the work function. Moreover, in a third step the control unit determines the operation signal in response to the operator control input and the load input.
According to another aspect of the present invention, a hydraulic system control unit is adapted to perform the method for controlling a hydraulic system adapted to perform at least one hydraulic work function in a working machine. According to another aspect of the present invention, a hydraulic system comprising the hydraulic system control unit is provided, and a working machine comprising the hydraulic system is provided.
An advantage available through an aspect of the present invention is that the operator will receive a feedback from the hydraulic system when loading the bucket. The operator is in focus without compromising the efficiency of the hydraulic system. The operator will feel the load weight acting on the bucket.
Such a feedback is in many cases very important for the operator when operating the machine. For instance, when operating an excavator it is vital when removing large, heavy objects (pieces of rocks etc.) to feel the weight of the object. Otherwise, the excavator may tip and/or break. In the same way, the operator of a wheel loader needs feedback from the hydraulic system when loading the bucket with heavy objects. Moreover, when loading gravel from a pile, the operation will be easier. The operator will for instance feel when the bucket is about to get stuck in the gravel pile.
Other preferred embodiments and advantages of the invention will emerge from the dependent claims and the detailed description below.